I installed Ubuntu 10 via DVD then immediately upgraded to 11 via the Software Center, but frequently (like at least 6 times this morning until I used the DVD to boot in rescue mode) the computer will boot, and log in without problems, but then it freezes. Either the screen just locks with the mouse pointer in place, or the screen goes black, or gray, but the computer doesn't respond to anything and I have to turn it off and start again. Any ideas as to what software or hardware problem this might be? My processor is an AMD Athlon II X4 620 on a GIgabyte motherboard (GA-MA785GMT-US2H) with 2GB of 1333 Mhz DDR3 RAM.